(a) Field of the Invention
The present invention relates to an active metal bed.
More particularly, the present invention relates to an apparatus aiming at recovery, storage and supply of hydrogen isotopes containing tritium gas, and to an active metal bed characterized by a constitution of a filter unit that makes it possible to flow a gas through an active metal without bringing about a large flowing resistance and a bypass flow while obtaining a high absorption velocity of tritium by a wide contacting area of gas, and characterized in that the tritium absorption velocity and thermal characteristic have been improved by enclosing a heat absorber aiming at an absorption and conduction of heat together with the active metal into the filter unit.
(b) Description of the Prior Art
Up to now, in order to recover a mixture of hydrogen isotope gases containing tritium (hereinafter referred to as "tritium" simply) from pure gas or a mixture with other gasses, a so called "active metal bed" in which plural stages of filter are placed in a sealed container and an active metal such as uranium is put on each filter stage has been used.
This will be explained as follows: In FIG. 1, 1 is an active metal; 2 is a filter; 3 is a sealed container; 4 is a heater; and 5 is an outer receptacle.
Tritium is absorbed and released by the reversible hydrogenation reaction of the active metal 1.
In the absorbing operation of pure hydrogen isotopes (tritium), tritium is introduced by an introducing tube 6 and absorbed to the active metal 1 on the filter shelf (a), (b), (c) in turn.
In case of intending to recover tritium from a gas mixture, tritium is absorbed into the active metal 1 while the gas mixture is flowing from the introducing tube 6 to a discharging tubing 7, and unabsorbed components are exhausted through the discharging tube 7.
This apparatus is used for the storage and supply of tritium as it is, where tritium is stored as a metal hydride and is released by heating the hydride.
In general, in an active metal bed, the active metal is enclosed in a filter for preventing its scattering since it is pulverized with the hydrogenation reaction. Moreover, the active metal cannot be filled up very high in the vertical direction within the filter because it is in danger of sintering by its weight. In order to secure the necessary capacity of tritium absorption, it is necessary that the active metal is filled up separately in plural stages, as described above, and a space is provided in the upper portion in each stages for absorbing the volume change of metal with the hydrogenation reaction.
Therefore, in the prior active metal bed, as shown in (a), (b) and (c) of FIG. 1, the active metal is placed on a shelf of plural stages of the filter 2. This structure is low in efficiency at the time of tritium absorption because the filter and metal powder having a large resistance to movement of gas and active metal powder are disposed in series along the gas flowing path from the introducing tube 6 to the discharging tube 7. That is, tritium entering from the introducing tube 6 is at first absorbed only on the under surface of shelf (a), and (b) and (c) do not act when (a) performs the absorption. Therefore, in case of urgent recovering of tritium, and other cases when recovery speed is essential, it is very inconvenient because only a portion of active metal contributes to absorption, and the recovering velocity is small. Moreover, after (a) is saturated with tritium, (b) commences absorption, but at this time gas has to pass through (a), which resists thereto. Further, after (a) and (b) are saturated, they block the movement of gas to (c).
Specially, in case of absorbing pure hydrogen isotope gas, this effect is so remarkable that, since the pressure in the interior of the bed decreases as the recovery goes forward and generally the conductance of filter and active metal powder decreases with the reduction of pressure, the movement of gas in low pressure is extremely obstructed and the absorption becomes slow. The disadvantageous point of this structure is the same also in case when gas flows through the bed. The pressure drop from the introducing tube 6 to the discharging tube 7 is so large that it is difficult to flow gas.
Moreover, the active metal bed as described above has such a disadvantageous point that the temperature of active metal rises due to the heat of hydrogenation reaction. Since all active metals used for the bed have a property such that the equilibrium pressure for hydrogen rises exponentially with the temperature, tritium absorption becomes insufficient for the equilibrium pressure increase when temperature rises. The generation of heat in the hydrogenation reaction is large and rapid, while the heat capacity of active metal powder is small and the heat conductivity is low since the powder is filled up coarsely. Therefore, in such an active metal bed, the generation of heat at the time of tritium absorption causes a rise in temperature of the active metal, and the absorption stops at the moment when the tritium partial pressure attains the equilibrium pressure at the elevated temperature, and thereafter the absorption of tritium proceeds at a low velocity as the active metal is cooled spontaneously.
On the other hand, in case of using the bed for supplying tritium, the sealed container 3 is heated, but the above described active metal bed has a structure such that the heat conductivity to active metal is not good. When heating the active metal, the heat from the heater 4 has to be conducted for a long distance from the outer wall of the sealed container 3 to the center radially through a sintered metal filter that is poor in heat conductivity. Moreover, neither the heat conductivity of the filter to active metal powder nor that between active metals is good. Consequently, it takes a long time before releasing of tritium, and it is feared that the active metal is superheated for poor temperature control so that the tritium pressure becomes excessive, or sintering of active metal and disconnection of heater occur due to local superheating.
The above defects are particularly remarkable in an active metal bed in which the tritium absorption capacity is large and in which, consequently, the amount of active metal to be filled up must be large. In a bed filled up with a large amount of active metal, the number of filling-up layer must be increased, for the height of filling-up cannot be increased. When arranging the layers in series, the resistance to absorption and flowing of gas is further increased. Moreover, since among materials constituting the bed, the rate of active metal increases and the rate of construction material and filter contributing to heat absorption and heat conductivity decreases relatively, the problem of heat generation and heat conductivity becomes more serious. In short, it is very difficult to make an apparatus that has large capacity with the structure of such active metal bed which has been hitherto used.
Such bed which has been hitherto used has a disadvantageous point in the aspect of tritium permeability. When tritium is released, the sealed container is heated to elevated temperatures, and at this time the amount of tritium permeating cannot be neglected because about 1 atmosphere of tritium exists. This tritium is collected into the outer receptable 5, and it has to be removed by gas purge, vacuum pumping, etc. The operation is complicated, and the loss of tritium is disadvantageous.